The present invention relates to the field of printed circuit boards and more specifically to a printed circuit board having flexible trace surfaces that allow the traces attached thereto to move in a direction of thermal expansion of components soldered to the traces.
Surface mount technology in general and ball grid array technology in particular are becoming increasing popular choices for integrated circuit packaging. Both the size and pin count of surface mount components continue to increase. This trend aggravates the problem of solder joint failure due to coefficient of thermal expansion mismatch between the printed circuit board and components mounted thereon. This problem is particularly acute for ball grid array components. Therefore, although the invention is applicable to any component packaging technology, it will be discussed using a ball grid array component as an example.
A bottom view of a ball grid array (BGA) component 10 is illustrated in FIG. 1. The underside of the BGA component 10 contains a plurality of solder bumps 12. Each solder bump 12 is electrically connected to an internal lead (not shown) which itself is connected to an integrated circuit formed on a silicon wafer (also not shown) inside the BGA component.
FIG. 2 illustrates the connection of a BGA component 10 to a printed circuit board 20. Each solder bump 12 is soldered to a corresponding trace pad 28 on the printed circuit board 20. As can be seen with reference to FIG. 5, the trace pad is an enlarged portion of the trace 27. Referring back to FIG. 2, the trace pads 28 are in positions corresponding to the positions of the solder bumps 12 on the BGA component 10. The other portions of the traces 27 are narrower to allow space for traces 27 between the trace pads 28. The solder bumps 12 may be attached to the corresponding trace pads 28 by well known methods such as reflow soldering or laser soldering.
When power is applied to an integrated circuit, some of that power is converted to heat by the movement of electrons through the integrated circuit. When integrated circuits are heated, they expand. The amount of expansion relative to the heat of a material is expressed as a quantity known as the coefficient of thermal expansion. The higher the coefficient of thermal expansion, the more a material expands when it is heated.
Referring back to FIG. 1, the physical center of the BGA component 10 is indicated by the point labeled NP. The BGA component 10 is perfectly symmetrical, therefore the physical center NP of the BGA is also the thermal neutral point NP. The neutral point NP is the point on the BGA component 10 from which all thermal expansion occurs in a radial direction. Thermal expansion directions are indicated by the vectors xe2x80x9cExe2x80x9d extending radially outward from the neutral point NP.
The amount of thermal expansion for each of the solder bumps 12 on the BGA component 10 is dependent upon the distance from the neutral point NP to the solder bump 12. This distance is known as the distance to neutral point, or DNP. As the DNP increases, the amount of movement of a solder bump 12 from the neutral point NP also increases. One reason that components are designed with square packages is to minimize the DNP for all connections.
Referring back to FIG. 2, the vectors xe2x80x9cExe2x80x9d indicate the direction of thermal expansion of the BGA component 10 from the neutral point NP. It should be appreciated that as heat is transferred from the BGA component 10 to the printed circuit board 20, the printed circuit board 20 also expands. However, because the printed circuit board 20 and the BGA component 10 are usually made of different materials, the corresponding coefficients of thermal expansion (CTE) may also be different, or mismatched. The result is a net force (which may be positive or negative, depending upon the respective CTEs of the BGA component and the printed circuit board and the amount of heat transferred to the printed circuit board by the BGA component) in the direction of the vector E on the solder bumps 12 and trace pads 28.
When leaded components (e.g. components with xe2x80x98Jxe2x80x99 leads or gull wing leads) are used, the leads act as compliant members, allowing for forces on solder joints caused by coefficient of thermal expansion mismatch. However, when leadless components such as BGAs are used, the solder bump 12 is the only available compliant member.
When component sizes and corresponding DNPs are small, the forces on the solder bumps 12 are also small and do not cause a problem. However, when component sizes and corresponding DNPs are large, the forces on the solder bumps 12 are also large and can lead to failure of the solder joint. FIG. 3 is an enlarged view of a single solder bump 12 that has failed. The force on the solder bump 12 in the direction of the vector xe2x80x9cExe2x80x9d caused by the CTE mismatch between the BGA component 10 and the printed circuit board 20 has caused cracks 14, 16 in the solder bump 12. A crack 14 has completely broken the connection between the solder bump 12 and the BGA component 10, resulting in an open circuit. A second crack 16 near the bottom of the solder bump 12 has also begun.
FIG. 4 illustrates one attempted solution to this problem. The spaces underneath the BGA component 10 between the solder bumps 12 and printed circuit board 20 are filled with an underfill material 18. The underfill 18 acts as an adhesive between the BGA component 10 and the printed circuit board 20 such that movement between them is prevented.
There are two main disadvantages to this solution. First, the adhesive eventually fails, leading to solder joint failure after repeated thermal cycling. Second, the underfill must be xe2x80x9cwickedxe2x80x9d under the BGA component 10 between the solder bumps 12, which is a time-consuming, and therefore expensive, procedure.
A second solution to the problem is to minimize the CTE mismatch between the BGA component 10 and the printed circuit board 12. The disadvantage to this solution is that the materials needed to achieve a good CTE match result in increased production costs.
What is needed is an inexpensive and reliable apparatus and method for attaching components and printed circuit boards with mismatched coefficients of thermal expansion.
The present invention solves the problem identified above by providing a circuit board with traces attached to a flexible trace surface such that the traces can be displaced in a direction of thermal expansion of a component attached to the traces without causing the failure of the solder joint between the component and the trace. In one embodiment, the printed circuit board substrate is etched away in areas not covered by the traces such that flexible protuberances are formed from the substrate underneath the traces. Methods for forming such a printed circuit board are also disclosed. In one method, a conductive layer is deposited on the printed circuit board substrate. The conductive layer is then etched to form conductive traces. The printed circuit board substrate is then selectively etched using the traces as a mask to form mesas which support the conductive traces.
In a second printed circuit board embodiment, a flexible layer of a silicone based material is deposited onto the printed circuit board substrate. As used herein, silicone based material means a material comprising at least approximately 50% silicone. The traces are then formed on top of the flexible silicone layer. The flexible silicone layer allows the traces to move in the direction of thermal expansion of an attached component without causing failure of the solder joint between the trace and the component.
It is known in the art to manufacture printed circuit boards with elastomeric layers between the traces and the underlying substrate. For example, Exacta Circuits Ltd. produces a product known as xe2x80x9cChipstratexe2x80x9d that has such an elastomeric layer. Known elastomeric layers are composed of standard materials commonly used in printed circuit board fabrication such as polyimides, epoxies, modified epoxies, nitrile rubber blends with phenolics, and acrylics. However, silicone based materials offer significant manufacturing advantages over these standard materials. For example, silicone does not have to be thermally cycled or mixed (in the case of two part epoxies) like these standard materials. Silicone based materials are also capable of being applied in much thinner layers (on the order of 1-2 mils) than these standard materials and can be applied in dry form as a laminate as well as in wet form.
The present invention has several advantageous features. First, circuit boards according to the present invention do not impose additional constraints upon circuit board layout. Second, circuit boards according to the present invention do not require any exotic materials or construction techniques. Third, the reliability of the solder joint is increased as compared with other techniques.
The above and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.